The present invention relates to a surface defect detecting apparatus for detecting a surface condition of an object to be checked such as a steel plate rolled by rolls.
Conventionally, it is known that rolled steel plates are made by a hot rolling or a cold rolling. Usually, surface defects made on the rolled steel plate may be divided into two types, one including scabs and scales which are caused by the property of the steel plate itself and the other including scars and the like which are made by the rolls in a rolling step, and these surface defects appear on the steel plate at random. Since, in particular, surface defects made on the steel plate by printing the scar or the like on the roll surface will appear periodically, the quality of the steel is remarkedly lowered. In practice, it has taken a relatively long time to detect the surface defect on the steel plate and stop the rolls so that a large amount of steel plate may be rolled uselessly. Therefore, it is required to check whether the roll itself has any defect or not and replace the roll without delay, if necessary. A surface defect detecting apparatus has been proposed which finds out a defect on the roll by inspecting a surface condition of the rolled steel plate on the assumption that a defect of the roll, if any, should appear substantially periodically on the surface of the rolled steel plate. In an example of such a surface defect detecting apparatus, the surface of the rolled steel plate is scanned by light, and the light reflected from the rolled steel plate is converted into an electric signal by a photoelectric converting device. The level of an output signal from the photoelectric converting device is determined in a level determination circuit. For example, when a signal at a level lower than a predetermined level, or an electric signal obtained when a defected portion is scanned, is received, a "1" signal is produced from the level determination circuit. The output signal from the determination circuit is applied to a plurality of shift register circuits connected in a cascade fashion. Each shift register circuit includes shift registers with a number of stages enough to store all the logic signals that are generated from the determination circuit during one rotation of the roll. Further, an output terminal of the last stage shift register of each of these shift register circuits are connected to an input terminal of an AND gate. In this surface defect detecting apparatus, when the " 1" signal is generated from the AND gate, it is judged that there is a defect on the roll. To be more specific, the judgment is based on an anticipation that, if there is any defect on the roll, the defect might periodically appear on the surface of the rolled steel plate and, therefore, the "1" signal indicating the presence of the defect on the rolled steel plate is stored in the same digit position, for example, in the last stage register of each shift register circuit, with the result that the "1" signal is generated from the AND gate. As a matter of fact, a slip occurs between the roll and the rolled steel plate and the steel plate may be stretched or contracted, a distance of the rolled steel plate transferred for each rotation of the roll is not constant. Therefore, the defects which appear on the rolled steel plate, resulting from the defect on the roll, are frequently detected at different timings in the scannings. The "1" signals indicating the presence of defects on the rolled steel plate are stored in the shift registers located at different digit positions of each shift register circuit. Consequently, it cannot be assured that all the "1" signals are transmitted to the last stage shift registers at the same time.